1. Field of the Invention
The present invention relates to a photoelectric conversion device and imaging system.
2. Description of the Related Art
These days, cameras have rapidly been digitized, and digital cameras have become popular. Digital cameras incorporate a photoelectric conversion device which photoelectrically converts an object image formed by an imaging optical system. Typical examples of the photoelectric conversion device are CCD and MOS photoelectric conversion devices. The MOS photoelectric conversion device includes a pixel array in which basic cells (pixels) including photoelectric conversion elements such as photodiodes are two-dimensionally arrayed, a capacitance unit which holds a signal from the pixel array, and a horizontal signal line for externally outputting a signal from the capacitance unit. A signal passing through the horizontal signal line is externally output from an output terminal.
Japanese Patent Laid-Open No. 2005-020483 discloses an arrangement in which a plurality of capacitive elements C1 to Cn are alternately connected to two horizontal signal lines HSL1 and HSL2 to multiplex signals passing through the two horizontal signal lines HSL1 and HSL2, as shown in FIG. 1 of Japanese Patent Laid-Open No. 2005-020483. In this arrangement, charge integrating amplifiers 14-1 and 14-2 are respectively connected to the output terminals of the two horizontal signal lines HSL1 and HSL2. Outputs from the charge integrating amplifiers 14-1 and 14-2 are alternately output to an A/D converter 16 via a multiplexer 15. According to Japanese Patent Laid-Open No. 2005-020483, one signal can be generated at a speed double the output speed of the charge integrating amplifiers 14-1 and 14-2, and input to the A/D converter 16.
Japanese Patent Laid-Open No. 2004-153705 discloses an arrangement in which odd-numbered memory cells 21 counted from the left in a line memory 20 are connected to a first horizontal signal line 22, and even-numbered memory cells 21 in the line memory 20 are connected to a second horizontal signal line 23, as shown in FIG. 1 of Japanese Patent Laid-Open No. 2004-153705. In this arrangement, a signal passing through the first horizontal signal line 22 is supplied to a first output amplifier 30a, and a signal passing through the second horizontal signal line 23 is supplied to a second output amplifier 30b. A switch 24 interposed between the first and second horizontal signal lines 22 and 23 connects them to each other in a correction mode. According to Japanese Patent Laid-Open No. 2004-153705, the signal voltage of the same pixel out of a plurality of pixels is supplied to each of a plurality of output amplifiers in the correction mode, obtaining amplifier outputs suitable for level correction.
However, according to the technique in Japanese Patent Laid-Open No. 2005-020483, sample-and-hold operation in multiplexing, that is, alternate output operation to the A/D converter 16 via the multiplexer 15 during the charge integrating operation by the charge integrating amplifiers 14-1 and 14-2 causes the waveform of a signal output to the A/D converter 16 to be distorted.
The number of output systems for outputting a signal (the number of output terminals via which a signal actually can be output) is sometimes subject to the limitation by the relationship with the outside of the photoelectric conversion device. However, the technique in Japanese Patent Laid-Open No. 2004-153705 does not disclose a read method when the number of output systems is subject to the limitation by the relationship with the outside of the photoelectric conversion device.
According to the technique described in Japanese Patent Laid-Open No. 2004-153705, signals of the same single pixel are output via different output amplifiers in the correction mode. Since the switch resistances differs between a plurality of output paths, the output signals may contain different offsets. This can cause degradation of an image such as a streak in the image.